This is a great video. It addresses some very advanced “internal” principles. By internal, I mean that they cannot be readily seen, so it is very difficult to accurately mirror the action in our minds. Chen Zhonghua does an excellent job of explaining by analogy then demonstrating so the viewer can see the physical action. Gradually, by watching and listening repeatedly, the viewer can move toward the ability to do what is being taught.
If there is a general theme, it might be separation of the yin and yang, but that is also primary taiji theme. In the Tailbone chapter, Shifu Chen does a “hands-on” demo that I had never seen before showing how the tailbone and buttocks split and go different directions. In the Full chapter, talks about He, harmony, which is an important element in the turning of the joints. In the Stretch chapter, he talks about the relationship between stretch and peng and how to generate peng. He shows martial, not push hands, applications to several moves in the form related to being full. In the Theory chapter, he discusses the relationships between stretching, peng and sinking the qi. These are hot topics in Taijiquan, qigong and internal training. They all require a mental element to evoke a physical response. Many practitioners only focus on the mental aspects. Shifu Chen discusses what needs to happen physically. The video is advanced in that these things are difficult to do, on the other hand, these are fundamental, foundational topics and a roadblock to many practitioners.